The pathways of mammalian cysteine metabolism are numerous and complex. More than a dozen distinct enzymes active toward cysteine have been identified, and the pathways initiated by these enzymes have been shown to lead to a multitude of intermediate products. Many of the pathways eventually intersect to either regenerate cysteine or to form a small number of catabolic products. The multiplicity and complexity of these pathways has hindered attempts to determine accurately the importance of individual pathways to the overall metabolism of cysteine. Further studies in this area would be greatly facilitated by techniques which allow selected pathways to be specifically inhibited in vivo and by the development of cysteine analogs which are metabolized by only a small number of the existing pathways. By reducing the number of metabolic pathways available, these techniques would substantially simplify the interpretation of in vivo experiments. The elucidation of mammalian cysteine metabolism through the design, synthesis and utilization in vivo of modified substrate and enzyme-specific inhibitors is the principal objective of the investigations proposed in this application. Although prospective inhibitors and substrate analogs will be tested with isolated enzymes, the investigations will center on studies with intact of surgically altered (e.g., nephrectomized) mice and rats. Attention will focus initially on the role of glutathione turnover in the oxidation of cysteine to cystine. Subsequent studies will define the quantitative importance of the several pathways of cystine catabolism. It is anticipated that the studies described will elucidate the relative importance of these pathwas in vivo and also indicate which organs or tissues are responsible for the metabolism observed.